1. Field of the Invention
The present invention relates to a hybrid integrated circuit device, and more particularly, to a hybrid integrated circuit device employing thermoplastic resin which sets within a short period of time, as well as to a method of manufacturing the hybrid integrated circuit device.
2. Description of the Related Art
In general, there are primarily two types of methods employed to seal a hybrid integrated circuit device in a resin package. One type of method is to seal a hybrid integrated circuit board having circuit elements, such as semiconductor elements, mounted thereon with a cap which is generally called a casing material. This sealing structure comprises a hollow structure or a resin-filled structure.
Another type of sealing method is a transfer molding method which is famous as a method of molding a semiconductor integrated circuit. The transfer molding method usually employs thermosetting resin. Under this method, a mold is heated to about 180.degree. C., and the thermosetting resin is set while being maintained at 180.degree. C. (hereinafter representing a phenomenon in which thermosetting resin is polymerized and solidified through thermal reaction) The thus-set mold(die) is removed from the mold, thereby completing a sealed semiconductor integrated circuit. Since solder used for mounting the circuit elements on the board is usually solder whose fusing temperature is high (hereinafter representing high temperature solder), the solder does not pose a problem such as fusing of solder.
However, a sealing structure using a casing material requires a margin to be ensured on a substrate in order to prevent the casing material from coming into contact with circuit elements provided in the casing material, thereby resulting in an increase in the outer dimension of the hybrid integrated circuit.
In contrast, as can be seen from the foregoing description, the transfer molding method requires a long period of time to set the thermosetting resin by application of heat to the resin. Consequently, it is impossible to improve the productivity of thermosetting resin.
The inventors of the present patent application took note of thermoplastic resin which does not require much time to set. The thermoplastic resin does not set and becomes fused when subjected to application of heat. Further, when being cooled, the thus-fused thermoplastic resin becomes solidified (hereinafter representing a phenomenon in which the thermoplastic resin is solidified without involving reaction). Accordingly, after molding of a hybrid integrated circuit, the thus-molded circuit is cooled, thus solidifying the thermoplastic resin. In this way, the hybrid integrated circuit can be sealed within a short period of time. However, in a case where a hybrid integrated circuit is sealed with thermoplastic resin through, e.g., injection molding, the thermoplastic resin is filled into the mold at a high temperature, e.g., a temperature of about 300.degree. C. As a result, the solder is fused, which in turn causes failures in electrical connection among the circuit elements mounted on the hybrid integrated circuit board.
To prevent this problem, it is only required to use high-temperature solder. However, in terms of prevention of deterioration insulating resin under a conductor pattern, low-temperature solder whose fusing temperature is low(hereinafter representing low temperature solder)is preferable. The present invention will now be described, provided that manufacturing an integrated circuit device using a solder which is fused at a temperature ranging from about 180 to 250.degree. C.
It is preferable to use a substrate having superior thermal conduction such as a metal substrate. However, in a case where such a metal substrate is used, the reverse side of the hybrid integrated circuit substrate becomes exposed, thereby posing a problem associated with insulation between the hybrid integrated circuit substrate and a chassis (a frame on which the hybrid integrated circuit device is mounted). Further, there is a problem associated with the moisture resistance of the hybrid integrated circuit: namely, entry of moisture into the hybrid integrated circuit substrate from a boundary surface between the exposed reverse side of the hybrid integrated circuit board and thermoplastic resin provided around the reverse side.
If a substrate having inferior thermal conduction, e.g., a printed board, a flexible sheet, a glass substrate, or a ceramic substrate, is used, solder is fused.
Further, in the case of full-molding of a hybrid integrated circuit through use of a transfer molding method, resin penetrates to the reverse side of an island of a lead frame. To this end, a gap is ensured between the reverse side of the island on which a chip is mounted and a mold, through use of pins or by the mold pinching the lead frame. However, under an injection molding method, thermoplastic resin is squirted at a pressure which is as high as 50 to 200 Kg/cm.sup.2. As a result, the lead frame becomes warped, or bonding wires are broken. A similar problem may arise not only in a lead frame but also in a hybrid integrated circuit board such as a metal board or a printed board.
The present invention has been conceived to solve the foregoing drawbacks of the prior art.